Surface Profile

Surface profile is determined by a range of variable which include the following:-

1. Strength and hardness of the surface being cleaned
2. The velocity of the abrasive
3. The hardness and specific gravity of the abrasive
4. The size or mass of the abrasive
5. The angle of abrasive impact
6. The particle shape of the abrasive
7. The time the surface is exposed to blasting

What is Hydro blasting?

A cleaning technique which relies on the energy of water striking a surface to achieve its cleaning effect.
Two different operating pressures are commonly used:

1. High pressure hydro blasting operating at pressures above 680 bar (10,000 psi)
2. Ultra high pressure hydro blasting operating at pressures above 1700 bar (25,000 psi)

What is water cleaning?

1. Low pressure water washing operates at pressure less than 68 bar (1000 psi)
2. High pressure water washing operates at pressure between 68-680 bar (1000-10,000 psi)
3. High pressure hydro blasting operates at pressure between 680-1700 bar
(10,000-25,000 psi)
4. Ultra high pressure hydro blasting operates at pressure above 1700 bar (25,000 psi)
normally in the range 2000-2500 bar (30,000-36,000 psi)

Advantages of Hydro Blasting Over Dry Abrasive Blasting

1. Health and Safety
No abrasive used, injury potential reduced
No respiration problems for operators from dust
Lower noise
No spark production
Minimizes disruption of other work in vicinity

2. Contamination
No dust contamination on wet paint, delicate machinery parts
No clean-up requirements

3. Surface Salt Reduction
Reduce salt levels on the surface

Soluble Salt Contamination

Salt contamination of steel and other substrates in ships' ballast and cargo tanks is a common cause of coating breakdown, via blistering. There are two well known mechanisms for blister development, osmosis and corrosion.

If the surface is contaminated before painting with hygroscopic material by more than a critical amount, then osmotic blister failure is likely to occur in service. Hygroscopic materials such as salts found in seawater tend to absorb water. They attract this water through the coating, where its volume forces blister formation. This type of failure frequently occurs within several weeks of immersion. The critical amount depends on a number of factors including coating characteristics and environmental factors. The more soluble the salts the more powerful the effect. Sodium chloride is much more soluble than sodium sulfate and is therefore a more harmful contaminant.

Breakdown by corrosion is a more gradual process, typically taking many months to appear. The presence of conductive salts under the film promotes corrosion. The corrosion products which are generated at the cathode have a powerful osmotic effect, giving rise to blisters, while the anode produces voluminous rust which also causes blisters.

The source of contaminant salts in ballast and cargo tanks can be from previous cargoes, ballast water, the atmosphere, washing water and also from blasting abrasive used to prepare the steel. Many ship owners are specifying salt contamination levels in their coating and new building contracts.



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